Electroplating methods are commonly used in numerous applications such as depositing metal films (copper interconnects) in semiconductor devices and forming magnetic layers in magnetic recording devices. Although magnetic layers in read and write heads may be deposited by a sputtering method, an electroplating process is usually preferred because the sputtering process produces a magnetic layer with a large magnetocrystalline anisotropy and higher internal stress. Electroplating is capable of generating a magnetic layer with a smaller crystal grain size and a smoother surface that leads to a high magnetic flux density (Bs) value and low coercive force (Hc).
In an electroplating process, an electric current is passed through an electroplating cell comprised of a working electrode (cathode), counter electrode (anode), and an aqueous electrolyte solution of positive ions of the metals to be plated on a substrate in physical contact with the cathode. By applying a potential to the electrodes, an electrochemical process is initiated wherein cations migrate to the cathode and anions migrate to the anode. Metallic ions such as Fe+2, Co+2, and Ni+2 deposit on a substrate (cathode) to form an alloy that may be NiFe, CoFe, or CoNiFe, for example. The substrate typically has an uppermost seed layer on which a photoresist layer is patterned to provide openings over the seed layer that define the shape of the metal layer to be plated. Once the metal layer is deposited, the photoresist layer and underlying seed layer are removed. The magnetic layers which become a bottom pole layer and top pole layer in a write head can be formed in this manner.
Magnetically soft materials in data storage are widely produced by electroplating from ferrous-based solutions. In the plating processes, ferrous ions are consumed by cathodic reduction reactions to form binary or ternary alloys such as NiFe, CoFe, and CoNiFe. However, ferrous ions are also converted to ferric ions either at the anode during plating or by homogeneous oxidation with dissolved oxygen. Formation of ferric ions can reduce plating current efficiency and adversely affect the surface morphology of the plated films. The presence of ferric ions can also result in poor plating thickness uniformity. In addition, the accumulation of ferric ions in the plating bath can lead to precipitation of ferric hydroxide within filters that remove particles from the electrolyte solution. As a result, mass transfer and/or solution flow to the plating cells is retarded. Ferric hydroxide can also be co-deposited into the plated films. High magnetic moment materials required for high areal density read/write heads are generally plated from a plating bath containing a high concentration of ferrous ions. Unfortunately, a high concentration of ferrous ions can accelerate the conversion process to cause an accumulation of ferric ions in the plating bath.
Conventionally, unwanted ferric ions can be reduced by periodically swapping aged plating solution. However, this practice is expensive because it creates hazardous waste and increases tool down time. Undesired ferric ions can also be suppressed by the addition of reducing agents such as trimethylamineborane (TMAB) as stated by T. Osaka, et al, Electrochemical and Solid State Letters, Vol. 6, No. 4, C53-C55 (2003). However, reducing agents that decompose during plating can be co-deposited into the plated films. The incorporation of decomposed components in a plated magnetic film can reduce the magnetic moment thereof due to dilution. The corrosion resistance of the plated film could also be lowered and cause defects in the resulting magnetic recording device. Another undesirable property of reducing agents is that they can interact with other chemical components in the plating bath and thereby cause changes in film composition and in the associated chemical-physical properties.
A method described in U.S. Patent Application Publication No. 2004/0217007 involves reducing ferric ion content in a plating solution by exposing hydrogen to an electrode that may be positioned in a plating cell or plating reservoir. However, Fe+3 content is only lowered by a few parts per million (ppm) per day using this technique.
In U.S. Pat. No. 5,932,082, a small amount of tartrate ions is added to a plating bath to prevent the precipitation of ferric hydroxide. However, this method does not address the need to convert unwanted ferric ions to ferrous ions.
A process for electroplating metals is disclosed in U.S. Pat. No. 5,173,170 in which a second anode that is insoluble is used to prevent metal build up in the plating bath. In a related Pat. No. Re. 34,191, an electroplating system comprised of an electrowinning cell having an insoluble anode, insoluble cathode, and a bath that communicates with the electroplating bath is described as a means of preventing metal build up. Unfortunately, there is no provision to reduce ferric ion content in the plating solution.
A method is described in U.S. Pat. No. 3,969,198 that slows the conversion of ferrous ions to ferric ions by oxidation. Additives such as sodium bisulphate, sodium benzene sulphinate and sodium para-toluene are employed for this purpose but may be depleted during the bath life. Additives can lead to other complications and require monitoring to ensure the proper concentration is maintained which leads to higher cost.
In U.S. Pat. No. 5,883,762, a cation-selective semi-permeable membrane is used to separate anode and cathode compartments and thereby block transport of oxidizable cations and anions to the anode. However, this method requires the additional activities of monitoring and manipulating the concentration of non-oxidizable plating cations in the anolyte and catholyte solutions. Therefore, an improved method of reducing ferric ions that does not involve reducing agents or modification of the electroplating cell is needed for ferrous based electroplating baths.